Manfishing

¹⁸While walking by the Sea of Galilee, he saw two brothers, Simon (who is called Peter) and Andrew his brother, casting a net into the sea, for they were fishermen. ¹⁹And he said to them, "Follow me, and I will make you fishers of men." ²⁰Immediately they left their nets and followed him. ²¹And going on from there he saw two other brothers, James the son of Zebedee and John his brother, in the boat with Zebedee their father, mending their nets, and he called them. ²²Immediately they left the boat and their father and followed him.
     - Matthew 4:18-22

Jesus calls Peter, Andrew, and the Sons of Thunder to follow him and he will make them "fishers of men." The obvious reference is to their prior occupation, that Jesus would employ them in pulling men into the boat instead of fish. Thomas Boston encourages us to take it one step further, considering why the men they were after would be compared to fish:

BUT WHY ARE UNCONVERTED MEN COMPARED TO FISH IN THE WATER?

Among other reasons, they are so because as the water is the natural element of fish, so sin is the proper and natural element for an unconverted soul. Take the fish out of the water, it cannot live; and take from a natural man his idols, he is ready to say with Micah, Ye have taken away my gods, and what have I more? The young man in the gospel could not be persuaded to seek after treasure in heaven, and lay by the world. It is in sin that the only delight of natural men is; but in holiness they have no more delight than a fish upon the earth, or a sow in a palace.

Oh, the woeful case of a natural man! Bless the Lord, O my soul, that when that was thy element as well as that of others, yet Christ took thee in his net, held thee, and would not let thee go, and put another principle in thee, so that now it is heavy for thee to wade, far more to swim in these waters.
     - Thomas Boston, The Art of Manfishing

The unconverted man loves sin and hates righteousness in much the same way that fish love the water and hate dry land. No amount of coaxing or persuasion can effectively convince a fish to forsake the water in favor of dry land. A fish may, for a short time, force himself to live out of the water. This will not last long. He may die, but more likely he will find a way back into the water for which he is most naturally suited. If a fish is to exist permanently out of the water, he must become something other than a fish.

This is what the Holy Spirit does in man's conversion. Through the proclamation of the Word (1 Cor. 1:23) and the working of the Holy Spirit (John 3:8), he who is spiritually dead is made something other than a dead man. The rebel who hates God and loves sin is made to love God and hate sin. Anything less than this and all you have is a convert in name only, who is no more suited for the Kingdom of God than a fish for dry land.